Fix toplevel docker build

[bts]

Hi there,

I had issues getting hydrachain building, so I had to make some changes to the (toplevel) Dockerfile and requirements.txt.

Additionally, I changed CMD so that docker run will start multiple nodes in a single process by default (I was getting this error with the default CMD of ["run"].)

Let me know if there's anything else you'd like me to change.

Thanks!
Brian